The present invention relates to ultra-wide band optical digital coherent detection.
For networks where transmission performance isn't as critical as cost per transmitted bit, the number of optical carriers for a fixed data rate is very important as fewer parallel modulation/detection operations is required to implement the transponder, therefore reducing the cost. One good example is the cost advantage of a 2×200G DP-16QAM system compared to 4×100G DP-QPSK for 400G transponder implementation by using higher order modulation format. Another way to reduce the number of optical carriers is to increase the base band electrical baud rate or symbol rate that is modulated onto the optical carrier. With higher symbol rate per carrier, the number of analog components such coherent mixers, lasers, and photodiodes can be reduced at the receiver side, lowering the implementation cost. For digital coherent transmission, however, this will push the BW requirements for ADC on the receiver higher than what is available for the commercial DSP chip.
For digital coherent transmission, receiver side DSP is used to compensate fiber impairments such as chromatic dispersion, polarization rotation, polarization mode dispersion, and fiber nonlinearity. Thus the standard method of time domain multiplexing, which is commonly used in recent publication for high symbol rate generation at the transmitter side, cannot be used at the receive side. Thus almost all high baud-rate transmission systems in recent literature use very high bandwidth ADC, in the form of digital sampling oscilloscope, with BW>30-GHz to achieve signal conversion at the receiver side. These ADC bandwidth requirements are typically more than double of what is available in commercial DSP. Thus it would require huge investment from the chip designers to implement the new ADCs with compromise in power consumption. It is unclear that with the added cost and power consumption in DSP, these high baud-rate transmission technologies will still be competitive in metro and datacenter networks.